Boiler Leaking Water: Where It's Coming From and What to Do
Water under the boiler, a drip from the pipe outside or a wet casing? How to make it safe, how to tell the causes apart by where the water appears, and when a leaking boiler needs an engineer the same day.
If your boiler is leaking water, switch it off at the controls, turn off the electrical supply to it if water is near any wiring, put a tray or towels underneath and check the pressure gauge. Then look at where the water is coming from. A drip from the copper pipe outside usually means the pressure relief valve is releasing high pressure. Water from the pipework under the boiler is often a loose or corroded fitting. Water from inside the casing points to a failed seal, pump or heat exchanger, and that needs a Gas Safe engineer.
Key Takeaways
- Switch the boiler off and keep water away from electrics before you do anything else.
- Where the water appears is the best clue: outside pipe, pipework under the boiler, or inside the casing.
- A dripping pressure relief pipe is a symptom of high pressure, not the fault itself.
- Leaks inside the casing are not a homeowner job. The casing should only be removed by a Gas Safe engineer.
- If you smell gas at any point, leave the boiler alone and call 0800 111 999.
If your boiler is leaking water, switch it off, keep water away from the electrics and put something underneath to catch the drips. Then check the pressure gauge and work out where the water is coming from, because that tells you most of what you need to know. A drip from the copper pipe outside is usually the pressure relief valve doing its job. Water around the pipes under the boiler is often a fitting. Water coming from inside the casing means a part inside has failed, and that is a job for a Gas Safe engineer.
What to do first
Switch the boiler off
Turn it off at the boiler controls. If water is running near the wiring, the control panel or the fused spur that feeds the boiler, switch it off at the spur as well, or at the consumer unit if you can't reach the spur without touching wet surfaces.
Catch the water
Put a tray, bowl or old towels under the leak. If water is getting into a ceiling below, move belongings, and if the ceiling is bulging keep people out of that room.
Read the pressure gauge
Note the figure. A reading near 3 bar points towards a pressure problem. A reading that has dropped towards zero tells you the system has lost a fair amount of water.
Isolate the water if it's pouring
On most combi and system boilers there are isolating valves on the pipes underneath. Turning the cold mains valve off stops fresh water feeding the boiler. If you can't find them, or the leak is serious, turn off the main stopcock.
If you smell gas at any stage, do not touch any switches. Turn off the gas at the meter if you can do so safely, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999. A water leak and a gas smell together should always be treated as a gas emergency first.
Where is the water coming from?
Dry the area and watch for a few minutes with a torch. Knowing where the first drop forms saves a lot of time on the phone and on the visit.
| Where you see water | Most likely cause | How urgent |
|---|---|---|
| Copper pipe outside the wall | Pressure relief valve releasing high pressure, or a worn valve | Fix within a day or two |
| Plastic pipe outside or to a drain | Condensate pipe, which drips in normal use | Usually normal |
| Pipework or valves under the boiler | Loose or corroded fitting, weeping isolating valve | Soon, depends on rate |
| From the base of the casing | Internal seal, pump, diverter valve or heat exchanger | Switch off, same-day visit |
| Down the wall behind the boiler | Pipe joint behind the boiler, or the flue in wet weather | Soon |
The plastic condensate pipe is meant to carry water away, so a drip from its end into a drain is normal. The copper relief pipe should be dry nearly all the time.
Common causes of a leaking boiler
1. High pressure and the pressure relief valve
Every sealed boiler has a pressure relief valve that opens at about 3 bar and sends water outside through a copper pipe. If you see water there and the gauge is high, the boiler isn't really leaking. It is protecting itself. The real fault is whatever pushed the pressure up, usually a filling loop left open or letting water through, an overfilled system, or an expansion vessel that has lost its charge. Our guide to boiler pressure that's too high explains how to bring it down. If the pressure is normal and the pipe still drips, the valve itself has probably worn, which we cover in pressure relief valve dripping.
2. Loose or corroded fittings under the boiler
The pipes under a boiler go through a lot of heating and cooling. Compression joints can work loose, and older isolating valves can start to weep from the spindle. You will usually see a bead of water forming on one joint, sometimes with green or white crusting around it. These are often straightforward repairs, but the joint may need remaking or the valve replacing rather than simply tightening.
3. Internal seals and the pump
Inside the casing, the pump, diverter valve and many of the pipe connections are sealed with rubber O-rings. Over years of heat and pressure these harden and start to leak. The water tends to collect in the bottom of the casing and drip from underneath the boiler. Because the leak is inside, near the electrics and the gas valve, it is not something to investigate yourself.
4. Corrosion and the heat exchanger
Sludge and corrosion in a heating system eat away at metal parts from the inside. On older boilers a corroded heat exchanger can start to leak, and that is often the point where a repair stops making financial sense. If your system has been running with dirty water for years, the engineer may also look at whether it needs cleaning. Our guide on signs your radiators need a power flush lists what to look out for.
5. Frost and the condensate pipe
In a cold snap the condensate pipe can freeze. The boiler then locks out, and water can back up and appear around the boiler. This is one of the most common winter call-outs, and it is covered in our article on a frozen or blocked boiler flue and condensate pipe.
Is a leaking boiler an emergency?
It depends on the rate and the place. Treat it as urgent and get someone out the same day if:
- Water is coming from inside the casing or dripping onto any electrics.
- The leak is steady enough to soak through to a ceiling or floor below.
- The boiler has lost pressure and you have no heating or hot water in cold weather, especially with elderly people, babies or anyone unwell in the house.
- You can see water and there are also signs of poor combustion, such as soot, a yellow flame or a CO alarm.
A slow drip from the relief pipe outside, or a weeping valve under the boiler that a bowl can cope with, can usually wait for a booked visit. It still needs fixing, because fresh water topped up into a sealed system brings in more corrosion. If you are not sure which side of the line you are on, our guide on when to call out a plumber urgently or wait may help.
What you can and can't do yourself
You can safely switch the boiler off, isolate the water, catch the leak, read the gauge, close a filling loop that has been left open and, on a cold system, bleed a radiator to lower high pressure. You can also top the pressure back up afterwards if the leak was only from the relief pipe and has stopped, using our steps on how to repressurise a boiler.
What the engineer will do
A good diagnosis starts with the questions above: where the water is, what the gauge reads cold and hot, and whether the leak is constant or only happens when the heating or hot water is running. The engineer will then:
- Check the pressure, the filling loop and the expansion vessel charge if the relief valve has been discharging.
- Dry the pipework and fittings under the boiler and run the system to find the exact joint or valve.
- If the leak is internal, remove the casing, find the failed seal or part and quote for the repair before doing it.
- Refill and repressurise the system, check for further leaks and test the boiler safely before leaving.
Our labour is £95 an hour and the £60 call-out is waived when the job runs over 2 hours. Parts depend on the boiler, so we confirm the full price once the cause is found and before any work starts. You can see how charges work on our pricing page, or read more about our boiler repair service.
Water coming from your boiler?
Tell us where the water is coming from and what the gauge reads. We'll tell you whether it can wait and confirm the price before any work starts.
📞 Call 020 7870 3200Or read about our boiler repairs and boiler pressure repairs in London.
Frequently Asked Questions
Is it safe to use a boiler that is leaking water?
It is best to switch it off. Water inside a boiler can reach the wiring and the control board, and a leak that is left running usually gets worse and can corrode parts that were fine. The one common exception is a slow drip from the pressure relief pipe outside with no other symptoms, where you can lower the pressure and keep the boiler running until an engineer visits.
Is a leaking boiler an emergency?
It is urgent when water is coming from inside the casing, dripping onto electrics, leaking steadily enough to damage ceilings or floors, or the boiler has lost pressure and left you without heating or hot water in cold weather. A slow drip from the relief pipe outside is a fault to fix in the next day or two rather than an emergency.
Why is my boiler dripping water from the pipe outside?
The copper pipe that runs through the wall is the discharge pipe from the pressure relief valve. It drips when the system pressure goes too high, usually because of an open or leaking filling loop, an overfilled system or a flat expansion vessel. It can also drip after the pressure is back to normal if the valve itself has worn and no longer seals.
Can a leaking boiler cause carbon monoxide?
Water leaking from the heating circuit does not produce carbon monoxide by itself. Carbon monoxide comes from poor combustion or flue problems. If your boiler is leaking and you also notice soot marks, a lazy yellow or orange flame, or your CO alarm sounds, switch it off, open windows, get outside and call the gas emergency line on 0800 111 999.
How much does it cost to fix a leaking boiler?
It depends on the part. Our labour is £95 an hour, and the £60 call-out is waived when the job runs over 2 hours. The engineer finds the source first and confirms the price of the repair and any parts before starting.
Sources and further reading
Source links checked 27 September 2026. Scheme rules and manufacturer instructions can change; use the current official guidance for your account or exact appliance.